1. Field of the Invention
The present invention relates to a vehicle attitude control device.
2. Discussion of the Related Art
Heretofore, as vehicle attitude control devices of this kind, there has been known a vehicle turning control device which is provided with wheel speed sensors, a steering wheel angle sensor, a yaw rate sensor and the like (refer to Patent Document 1). In this vehicle turning control device, an arithmetic circuit 58 in an ECU 28 calculates a target yaw rate γt to control a yawing moment based on each of detection values which are detected by those various sensors 50. A control circuit 64 controls a controlled object 70 (automatic brake system) so that the detected actual yaw rate γ coincides with the target yaw rate γt. In the arithmetic circuit 58, an operation section 58b calculates a stability factor. More specifically, in the operation section 58b, a stability factor “A” is obtained from the following expression 1.A=((V−δ)/(γ−L)−1)/V2  (Expression 1)
As apparent from the above expression 1, a calculated stability factor A in this case is obtained based on a vehicle body speed V, a steering angle δ of front wheels and an actual yaw rate γ. Accordingly, the stability factor calculated here is not a stability factor provided in advance as a unique or peculiar value for the vehicle 1, but is an estimated stability factor as a result of the calculation on the basis of an actual stable turning motion of the vehicle 1, that is, is a calculated stability factor.
And, in the arithmetic circuit 58, a stability factor in a steady gain 58a is learned from the calculated stability factor. The target yaw rate γt is calculated by making this learned stability factor reflect on the steady gain 58a. Therefore, even when the stability factor of the vehicle 1 varies, a suitable target yaw rate is calculated all the time of such variation.
Therefore, in this vehicle turning control device, a stability factor is estimated based on an actual turning situation of the vehicle, and thus, it can be realized to calculate a more exact motion state compared with the case where a motion state of the vehicle is calculated by using the stability factor fixed to a predetermined value. That is, it can be realized to calculate a yaw rate deviation which is a difference between the target yaw rate γt and the actual yaw rate γ as the motion state of the vehicle.                [Patent Document 1] Japanese unexamined, published patent application No. 10-258720 (pp. 5 and 6, FIG. 3)        
However, in the aforementioned vehicle turning control device, although the estimated stability factor is calculated on the basis of a stable turning motion, it has the possibility of differing from the actual stability factor of the vehicle. In this case, in the relation between the estimated stability factor and a stability factor exerted really on a vehicle (hereafter as an actual stability factor), there exist domains where the control is properly executed and other domains where the control is hard to be properly executed or is not executed.
More specifically, since the estimated stability factor almost coincides with the actual stability factor in the domains A1, A2 and A3 as shown in FIG. 14, the control (i.e., oversteer or understeer) is properly executed. On the other hand, in the domains A4, A5 and A6, since the actual stability factor is small, the vehicle is in an oversteer tendency (state). Accordingly, the actual yaw rate tends to increase. In addition, since the estimated stability factor is large than the actual stability factor, the target yaw rate tends to decrease excessively. This results in increasing the absolute value of the yaw rate deviation. That is, the absolute value of the yaw rate deviation is larger than the absolute value of the yaw rate which the vehicle has inherently, so that it is apt to exceed a fixed control intervention threshold value (control intervention threshold value to execute the OS control). Accordingly, it is likely that the oversteer control is executed by mistake notwithstanding that the vehicle is in a state that the oversteer control (the OS control) should not be executed. In another domain A6, such a tendency appears more remarkably since the estimated stability factor is further larger than the actual stability factor compared with those in the domains A4 and A5.
Furthermore, in the domains A7, A8 and A9, since the actual stability factor is large, the vehicle is in the understeer tendency (state). Accordingly, the actual yaw rate tends to decrease. In addition, since the estimated stability factor is smaller than the actual stability factor, the target yaw rate increases excessively. This results in increasing the absolute value of the yaw rate deviation. That is, the absolute value of the yaw rate deviation is larger than the absolute value of the yaw rate which the vehicle has inherently, so that it is apt to exceed a fixed control intervention threshold value (control intervention threshold value to execute the US control). Accordingly, it is likely that the understeer control is executed by mistake notwithstanding that the vehicle is in a state that the understeer control (the US control) should not be executed. In another domain A9, such a tendency appears more remarkably since the estimated stability factor is further smaller than the actual stability factor compared with those in the domains A7 and A8. In any case, in the domains A4 to A9, it is likely that the attitude control is executed excessively (more than as needed).
In addition, the reason why the target yaw rate decreases as mentioned above is that the target yaw rate decreases as the stability factor increases, because the target yaw rate Tω is calculated based on the following expression 2. Conversely, the reason why the target yaw rate increases is that the target yaw rate increases as the stability factor decreases. Symbol V denotes a vehicle body speed, symbol ξ denotes a steering angle of the vehicle, and symbol L denotes a wheelbase of the vehicle.Tω=(V×ξ)/(L×(A×V2+1))  (Expression 2)